The present invention relates to amplification of optical signals.
Erbium/Ytterbium (Er/Yb) cladding pumped optical amplifiers are capable of producing amplifiers with high output powers. It is difficult to make an efficient Erbium (Er) only cladding amplifier because of the low pump absorption of Erbium as compared to the high pump absorption of Er/Yb. Erbium-only single mode amplifiers have extremely good low noise properties and can operate at low input powers. For a low noise, high output power amplifier, a common strategy is to construct a hybrid amplifier consisting of a single mode Er pre-amplifier followed by a high power Er/Yb co-doped amplifier. This type of combination has both the low noise properties of Er amplifiers and the high output power capabilities of Er/Yb amplifiers. However, this dual amplifier requires a single mode pump laser for the Er pre-amp and a separate high power pump laser for the high power post amplifier. It also requires two wavelength division multiplexers for combining the two pump lasers into the amplifier. As a result, this optical amplifier design is expensive.
It is desired to provide a lower cost optical amplifier that has many of the good low noise properties of an Er single mode amplifier and the high output power properties of Er/Yb amplifiers.